Oil-based drilling fluid is a drilling fluid with oil as a continuous phase. Compared with water-based drilling fluid, oil-based drilling fluid has such advantages as strong inhibitive ability, good lubricity, resistance to salt contamination, resistance to calcium contamination and small damage to oil and gas reservoir. Obviously, the strong inhibitive ability, strong lubricity and good reservoir protection capability of oil-based drilling fluid make it more suitable to well drilling of building up sections and long horizontal sections. However, the prior art of oil-based drilling fluid is still not perfect, as mainly manifested in the following few aspects.
(1) Inadequate high temperature resistance property: with the increase of well depth and the rise of bottom well temperature, the requirement for temperature resistance of oil-based drilling fluid increases, too.
(2) Universally low density: oil-based drilling fluid with ordinary density no longer can meet the requirements of high pressure formation for the density of drilling fluid, while raising density of drilling fluid will bring about problems like poor rheological behavior.
The clay-phase oil-in-water drilling fluid improves the suspension property of the oil-based drilling fluid system mainly through organic clay. Typical composition is: base oil or synthetic base oil serving as an external phase; organic salt, dissolved in internal phase water (typically, is a 30% CaCl2 aqueous solution); other additives, such as suspending agent, filtrate reducer, humectant, emulsifier and rheological modifier. As organic clay is used as a clay phase, the clay-phase oil-in-water drilling fluid has the problems of poor temperature resistance, large damage to reservoir and low density. In comparison, the current clay-free oil-in-water drilling fluid uses rheological modifier to replace the clay phase in the clay-phase oil-in-water drilling fluid. As a result, the drilling fluid has the advantages of resistance to high temperature, low damage to reservoir and high density, but the current clay-free oil-in-water drilling fluid still can hardly obtain super high temperature resistance and super high density and realize exploitation of oil and gas from high pressure deep wells.